Cryogenic Air Separation Method and Air Separation Unit

ABSTRACT

According to the present invention, a method for cryogenic separation of air using an air separation unit comprising a rectification column is provided. Feed air is compressed, cooled and rectified in the rectification column obtaining an overhead gas, wherein a part of the overhead gas of the rectification column is condensed using fluid withdrawn from the rectification column, wherein the condensed overhead gas is used at least in part as a liquid reflux to the rectification column, wherein a first part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, compressed and reintroduced into the rectification column, and wherein a second part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, expanded and withdrawn from the air separation unit.

The present invention relates to a method for cryogenic separation of air and to a corresponding unit according to the preambles of the independent claims.

PRIOR ART

The production of air products in liquid or gaseous form by means of cryogenic separation of air in air separation units is known and described for example in H.-W. Haring (Ed.), Industrial Gases Processing, Wiley-VCH, 2006, in particular in Section 2.2.5, “Cryogenic Rectification”.

Air separation units are equipped with rectification column systems which are traditionally designed as two-column systems, in particular as classic Linde double-column systems, but also as three- or multi-column systems. In addition to the rectification columns for obtaining nitrogen and/or oxygen in liquid and/or gaseous state, i.e. the rectification columns for nitrogen/oxygen separation, further rectification columns may be provided for obtaining further air components, in particular the noble gases krypton, xenon and/or argon.

The rectification columns of classical rectification column systems are operated at different pressure levels. Known double column systems comprise a so-called high pressure column (also called pressure column, medium pressure column or lower column) and a so-called low pressure column (also called upper column). The high-pressure column is typically operated at a pressure level of 4 to 7 bar, in particular approx. 5.3 bar. The low-pressure column is operated at a pressure level of typically 1 to 2 bar, in particular approx. 1.4 bar. In certain cases, higher pressure levels can also be used in both rectification columns. The pressures indicated here and below are absolute pressures at the top of the respective columns.

Depending on the desired product spectrum, alternatives to classical air separation plants and their rectification column systems may be more attractive. One of these is the so-called SPECTRA process, as described in EP 2 789 958 A1 and other patent literature cited therein. In its simplest form, such a process uses a single rectification column. A liquid reflux to the main (or single) rectification column is provided by condensing a part of its overhead gas in a heat exchanger. Fluid from the same rectification column is used for cooling in the heat exchanger that condenses the overhead gas. Parts of the fluid used for cooling are compressed by means of one or more cold compressors and are returned to the same rectification column thereafter. Further parts of the fluid used for cooling are expanded in one or more expansion turbines and are typically vented to the atmosphere or used as regenerator gas. The cold compressor(s) and the expansion turbine(s) may be coupled.

With SPECTRA processes, very favorable air factors may be achieved, i.e. large quantities of product per quantity of air used, and a high yield of nitrogen is possible. Below, a corresponding process will be explained in more detail. The term “SPECTRA process” is intended to refer to the single-column process for nitrogen recovery described above or a modified single-column process in which, as also explained below, an additional rectification column is used for oxygen production. The term may therefore be replaced at any time by the term “single-column process or modified single-column process”, and it is characterized by features mentioned above, and mainly by the fact that overhead gas from a main rectification column is cooled and at least partly condensed using fluid from the same rectification column.

A classical SPECTRA process is shown in FIG. 1 and is further explained below. Particularly in large plants, as shown in FIG. 1 , often three expansion turbines are used, two of which are typically embodied as so-called TFC turbines. The abbreviation TFC stands for Turbine, Friction, Compression, i.e. corresponding turbines are coupled with a friction brake, typically an oil brake, and a compressor, respectively. The latter compressors are used as the cold compressors mentioned above. The reason for using two TFC turbines is mainly the result of feasibility limits. The third expansion turbine is typically coupled with a generator, in order to increase flexibility and to enable the production of larger amounts of liquid nitrogen as well.

However, providing three expansion turbines significantly increase capital and operating expenses (CAPEX/OPEX). This latter is particularly the result of the relatively small sizes of expansion turbines and cold boosters. The direct mechanical coupling of the expansion turbines and the cold boosters requires speed adjustments which are not always optimal in terms of efficiency.

The object of the present invention is therefore to provide an advantageous solution for such cases, i.e. a solution which reduces costs and increases efficiency particularly in large SPECTRA processes which classically require three expansion turbines. A further object of the present invention is to maintain flexibility without requiring a further unit to be additionally activated or deactivated.

DISCLOSURE OF THE INVENTION

Against this background, the present invention proposes a process for the cryogenic separation of air and a corresponding air separation unit with the features of independent claims. Preferred embodiments of the present invention are the subject of the dependent claims and of the description that follows.

Before explaining the characteristics and advantages of the present invention, some basic principles of the present invention are explained in more detail and terms used in describing the invention will defined.

The devices used in an air separation plant are described in the cited technical literature, e.g. Haring (see above) in Section 2.2.5.6, “Apparatus”. Unless the following definitions deviate from this, explicit reference is therefore made to the technical literature cited for the use of the language used in the context of this application.

Liquids and gases can be rich or poor in one or more components in the language used here, where “rich” can mean a content of at least 75%, 90%, 95%, 99%, 99.5%, 99.9% or 99.99% and “poor” can mean a content of at most 25%, 10%, 5%, 1%, 0.1% or 0.01% on a mole, weight or volume basis. The term “predominantly” may correspond to the definition of “rich”. Liquids and gases may also be enriched or depleted in one or more components, these terms referring to a content in a starting liquid or gas from which the liquid or gas was derived. The liquid or gas is “enriched” if it contains at least 1.1 times, 1.5 times, 2 times, 5 times, 10 times, 100 times or 1,000 times the content, and “depleted” if it contains at most 0.9 times, 0.5 times, 0.1 times, 0.01 times or 0.001 times the content, based on the starting liquid or gas, of a corresponding component. If, for example, the term “oxygen”, “nitrogen” or “argon” is used here, this may also mean a liquid or a gas which is rich in oxygen or nitrogen, but which does not necessarily have to consist exclusively of this.

The present application uses the terms “pressure level” and “temperature level” to characterize pressures and temperatures, which are intended to express that corresponding pressures and temperatures in a corresponding installation need not be used in the form of exact pressure or temperature values in order to realize the inventive concept. However, such pressures and temperatures typically move in certain ranges, such as ±1%, 5%, 10% or 20% around an average value. Corresponding pressure levels and temperature levels can lie in disjunctive areas or in areas that overlap each other. In particular, pressure levels include, for example, unavoidable or expected pressure losses. The same applies to temperature levels. The pressure levels given here in bar are absolute pressures.

An expansion turbine or turbo expander can be coupled with a friction brake and/or a compressor, as mentioned hereinbefore. Also the compressor can be embodied as a turbo compressor in particular. A combination of a turbo expander and a turbo compressor is also referred to as a “turbine booster” in the art. In a turbine booster, the turbo expander and the turbo compressor are mechanically coupled, whereby the coupling can take place at the same speed (e.g. via a common shaft) or at different speeds (e.g. via a suitable transmission or gearbox).

The term “cold compressor”, as used herein, shall refer to a compressor to which a fluid flow at a temperature level well below 0° C., in particular below −50, −75 or −100° C. and as low as −150 or −200° C. is supplied. A corresponding fluid flow is cooled to a corresponding temperature level in particular by means of the main heat exchanger of the air separation unit (see immediately hereinbelow).

A “main air compressor” is characterized by the fact that it compresses the entire air supplied to, and separated in, the air separation unit. In one or more additional compressors which are optionally provided, also referred to as booster compressors, a portion of the air previously compressed in the main air compressor may be further compressed. Accordingly, the “main heat exchanger” of an air separation plant is the heat exchanger in which at least the majority of the air supplied to and separated in the air separation plant is cooled. This takes place at least in part in counterflow to fluid streams that are discharged from the air separation plant. Fluid streams or “products” which are “discharged” from an air separation plant are, in this context, fluids that no longer participate in internal plant circuits but are permanently removed from them.

A “heat exchanger” for use within the scope of this invention may be designed in a manner customary in the field. It serves for the indirect transfer of heat between at least two e.g. countercurrent fluid streams, e.g. a warm compressed air stream and one or more cold fluid streams or a cryogenic liquid air product and one or more warm or warmer, but possibly also cryogenic fluid streams. A heat exchanger may consist of one or more heat exchanger sections connected in parallel and/or in series, e.g. one or more plate heat exchanger blocks. It is, for example, embodied as a Plate Fin Heat Exchanger (PFHE). Such a heat exchanger has “passages” which are designed as separate fluid channels with heat exchange surfaces and are connected in parallel and separated by other passages to form “passage groups”. In a heat exchanger, heat is exchanged at any time between two mobile media, namely at least one fluid stream to be cooled and at least one fluid stream to be heated.

A “condenser evaporator” is a heat exchanger in which a first, condensing fluid flow is in indirect heat exchange with a second, evaporating fluid flow. Each condenser evaporator has a condensing space and an evaporation space. The liquefaction and evaporation chambers have liquefaction and evaporation passages respectively. In the condensing chamber the condensation (liquefaction) of the first fluid stream is carried out, in the evaporation chamber the evaporation of the second fluid stream. The evaporation chamber and the condensing chamber are formed by groups of passages which are in heat exchange relationship with each other.

The relative spatial terms “above”, “below”, “next to”, “side by side”, “vertical”, “horizontal” etc. refer here to the spatial orientation of the rectification columns of an air separation plant in normal operation. An arrangement of two rectification columns or other components “above” the other here means that the upper end of the lower of the two apparatus parts is at a lower or the same geodetic height as the lower end of the upper of the two apparatus parts and the projections of the two apparatus parts overlap in a horizontal plane. In particular, the two parts of the apparatus are arranged exactly one above the other, i.e. the axes of the two parts of the apparatus run on the same vertical straight line. However, the axes of the two apparatus parts can also be offset from each other, especially if one of the two apparatus parts, for example a rectification column or a column part with a smaller diameter, is to have the same distance to the sheet metal shell of a cold box as another with a larger diameter.

Advantages of the Invention

As with other cryogenic air separation processes, in the SPECTRA process air is compressed, pre-purified and cooled to a temperature suitable for rectification, which is normally at or near its dew point. The air is then fed into a rectification column wherein it is rectified under the typical pressure of a high-pressure column as described above, obtaining a overhead gas enriched in nitrogen compared to atmospheric air and a bottom liquid enriched in oxygen compared to atmospheric air. As mentioned above, the rectification column operated accordingly may be the only rectification column in a SPECTRA process, as it is the case in the embodiments discussed below. However, one or more further rectification columns, particularly for oxygen production, may be present as well. This can also be the case according to the present invention.

In a SPECTRA process, and according to the present invention, a part of the overhead gas from the rectification column is condensed using fluid, in particular cryogenic liquid, which is withdrawn from the same rectification column. The condensed overhead gas is at least in part used as a liquid reflux to the same rectification column. A first part of the fluid which is used for cooling is, after its use for cooling, compressed and reintroduced into the same rectification column from which it was withdrawn. A second part of the fluid which is used for cooling is, after its use for cooling, expanded and withdrawn from the air separation unit, particularly after heating it to ambient temperatures and partly using it as a regeneration gas. As mentioned, the compression and the expansion of the first and the second part of the fluid, respectively, is classically done using arrangements wherein at least on expansion turbine is coupled to at least one compressor. However, this requires, as mentioned before, that more than one expansion/compression unit is used in larger air separation units.

The “first” and “second” parts of the fluid which is withdrawn from the rectification column and which is used for cooling the overhead gas may particularly be withdrawn from the rectification column in form of two separate liquid streams of which a first stream corresponding to the first part of the fluid that is compressed and reintroduced into the rectification column may particularly comprise a larger nitrogen content than a second stream corresponding to the second part of the fluid that is expanded and removed from the air separation unit. However, in general, also one stream may be withdrawn from the rectification column which may then, before or after the use for cooling, be divided into portions corresponding to the first and second parts.

In summary, the present invention proposes a method for cryogenic separation of air, using an air separation unit comprising a rectification column, wherein feed air is compressed, cooled and rectified in the rectification column obtaining an overhead gas, wherein a part of the overhead gas of the rectification column is condensed using fluid withdrawn from the rectification column, wherein the condensed overhead gas is used at least in part as a liquid reflux to the rectification column, wherein a first part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, compressed and reintroduced into the rectification column, and wherein a second part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, expanded and withdrawn from the air separation unit. The invention thus relates to a SPECTRA process, as mentioned.

According to the present invention, for compressing the first part of the fluid which is used for cooling the overhead gas of the rectification column, a compressor which is coupled to an electric motor via a first gearbox is used, and for expanding the second part of the fluid which is used for cooling the overhead gas of the rectification column an expansion turbine which is coupled to an electric generator via a second gearbox is used. The first gearbox and the second gearbox are identically designed.

The term “identically designed”, as used herein, shall particularly refer to, or include, identical transmission ratios, i.e. reduction or multiplication ratios, in the gearboxes, which particularly may be realized by using identical internal gearbox components or specifications for the internal gearbox components including at least one of identical sprocket diameters, an identical number of teeth of sprockets and identical shaft diameters. In the language used herein, “identical reduction or multiplication ratios” are present when a reduction rate in one unit is the same, or the inverse, of a multiplication rate in the other unit and vice versa. This applies for elements of the same function in the first and second gearbox when compared between the identically designed gearboxes. Identically designed gearboxes not necessarily are operated, or designed to be operated, in identical directions of rotation, but this is the case in a particularly preferred embodiment of the present invention. When operating gearboxes with identical directions of rotation, or designing them to be operated in this way, identical bearing pad assemblies can be used.

Using identically designed (in the sense just explained) or even (besides mechanical tolerances and the like) mechanically identical gearboxes provides the specific advantage that a large number of mutually interchangeable components can be used. This particularly results in the reduction of the number of spare parts to be kept in stock. For exchanging gearboxes in cases of failure, only one spare gearbox has to be kept in stock. Further advantages include a simplified maintenance because of identical maintenance routines or tools, and a significant cost reduction. Using uncoupled expansion turbines and compressors according to the present invention allows for using only one compressor instead of two or more, because no friction brakes, which may be limited to smaller sizes, are used. Compression and expansion can be done largely independently, obviating the need for a further generator turbine which is used, in classical arrangements, to provide for such flexibility.

As mentioned, the first part of the fluid which is used for cooling the overhead gas of the rectification column may be a cryogenic liquid withdrawn from the rectification column at a first position, and the second part of the fluid which is used for cooling the overhead gas of the rectification column may be a cryogenic liquid withdrawn from the rectification column at a second position. In this connection, the first position may be above the second position and/or the second position may particularly correspond to a position at the bottom of the rectification column. Furthermore, the first part of the fluid which is used for cooling the overhead gas of the rectification column may particularly have a higher nitrogen content than the second part.

According to a particularly preferred embodiment of the present invention, electric energy generated in the generator is at least in part used to operate the motor. The compressor and the expansion turbine may be operated at identical speed settings.

The present invention also relates to an air separation unit comprising a rectification column, the air separation unit being adapted to compress, cool and rectify feed air in the rectification column obtaining an overhead gas, wherein means are provided which are adapted to condense a part of the overhead gas of the rectification column using fluid withdrawn from the rectification column, wherein means are provided which are adapted to use the condensed overhead gas at least in part as a liquid reflux to the rectification column, wherein means are provided which are adapted to compress and reintroduce into the rectification column a first part of the fluid which is used for cooling the overhead gas of the rectification column after its use for cooling, and wherein means are provided which are adapted to expand and withdraw from the air separation unit a second part of the fluid which is used for cooling the overhead gas of the rectification column after its use for cooling.

According to the present invention, for compressing the first part of the fluid which is used for cooling the overhead gas of the rectification column a compressor which is coupled to an electric motor via a first gearbox is provided, for expanding the second part of the fluid which is used for cooling the overhead gas of the rectification column an expansion turbine which is coupled to an electric generator via a second gearbox is provided, and the first gearbox and the second gearbox are identically designed.

As to further embodiments and advantages of a corresponding air separation unit, reference is made to the explanations above relating to the inventive methods and their embodiments. This is also the case for an air separation unit according to a particularly preferred embodiment which is adapted to perform a method according to the invention or an advantageous embodiment as explained hereinbefore.

Further details of the invention will be described with reference to the appended drawings which illustrate an embodiment of the invention vice versa the prior art.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an air separation unit not forming part of the invention.

FIG. 2 shows an air separation unit according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the figures, components with comparable or identical function are indicated with like reference numerals. A repeated explanation is omitted for reasons of conciseness only.

FIG. 1 shows an air separation unit not forming part of the invention in the form of a simplified, schematic process flow diagram. The air separation unit is indicated with 90.

In a compression unit 1 of the air separation unit 90, which comprises three parallel compression lines including compressors or compressor stages with aftercoolers, respectively, an amount of feed air taken from the atmosphere A is compressed and provided as a feed air stream a. The feed air stream a is cooled in a direct contact cooling unit 2 with water W and, still indicated a, supplied to a purification unit 3 which, in the embodiment illustrated, comprises two adsorber lines each containing two adsorption vessels.

The purified feed air stream, still indicated a, is then cooled in a main heat exchanger 4 of the air separation unit 90. The feed air stream a is withdrawn from the main heat exchanger 4 close to its cold end and is then introduced into a rectification column 5 where the air is rectified obtaining an overhead gas and a bottom liquid.

From the top of the rectification column 5, its overhead gas is withdrawn as a fluid stream b. From the fluid stream b, a part is heated, in the form of a fluid stream c, in the main heat exchanger 4 and e.g. provided to provide products C1, C2 of the air separation unit 90. A further part is condensed in the form of a stream c in a heat exchanger 8 using fluid withdrawn from the rectification column 5 in form of fluid streams d and e. The fluid streams d and e form first and second parts of the fluid which is used for cooling the overhead gas of the rectification column 5 and are a cryogenic liquid withdrawn from the rectification column 5 at a first position on the one hand (first part), and cryogenic liquid withdrawn from the rectification column 5 at a second position below the first position (or from the bottom) on the other hand (second part). The fluid streams d and e are slightly cooled in the main heat exchanger 4 before they are introduced into the heat exchanger 7.

The fluid stream d is, after its use for cooling in the heat exchanger 7, at least in part compressed in compressors 91 and 92 which are arranged in parallel and which are coupled to friction brakes 93 and 94 and expansion turbines 95 and 96 which are also arranged in parallel, respectively. A further expansion turbine 97 is arranged in parallel therewith and is coupled to an electric generator G. A part of the fluid stream d may be vented to the atmosphere A. After compression in the compressors 91, 92, the fluid stream which is still indicated with d, even if parts thereof might be vented to the atmosphere or used otherwise, is cooled in the main heat exchanger 4 and is thereafter in the air separation unit 90 reintroduced into the rectification column 5.

The fluid stream e is, after its use for cooling in the heat exchanger 7, further heated in the main heat exchanger 4 and then expanded in parallel in the expansion turbines 95 and 96 and optionally as well in the further expansion turbine 97 and is then warmed in the main heat exchanger 4 and withdrawn from the air separation unit 100. A part can be used at least temporarily as a regenerating gas for the purification unit 3.

As further illustrated in FIG. 1 , a part of the overhead gas of the rectification column 5 which was condensed in the heat exchanger 8 may be subcooled in a subcooler 9 and may be supplied, in the form of a fluid stream g, as a liquid nitrogen product G. A part of the condensed overhead gas of the rectification column 5 expanded for this subcooling may be combined with the fluid stream e after its expansion and may be heated together therewith. A further part of the condensed overhead gas of the rectification column 5 is supplied to the rectification column 5 as a liquid reflux in the form of a fluid stream h. A yet further part may be used to form a purge stream p and a purge P. A part of the feed air stream a may be bypassed to the compressors 91 and 92. In form of a fluid stream i, liquid nitrogen I may be introduced into the unit 90.

FIG. 2 shows an air separation unit according to an embodiment of the present invention in the form of a simplified, schematic process flow diagram. The air separation unit is indicated with 100. Only features differing from the air separation unit 90 according to FIG. 1 are described hereinbelow.

Instead of parallel compressors 91, 92 which are part of the air separation unit 90 according to FIG. 1 , only one compressor 6 is provided. Likewise, instead of the parallel compressors 95 to 97, only one expansion turbine 7 is provided. The compressor 6 is coupled to an electric motor M via a first gearbox 61. The expansion turbine 7 is coupled to an electric generator G via a second gearbox 71. The first gearbox 61 and the second gearbox 71 are identically designed. 

1. A method for cryogenic separation of air, using an air separation unit comprising a rectification column, wherein feed air is compressed, cooled and rectified in the rectification column obtaining an overhead gas, wherein a part of the overhead gas of the rectification column is condensed using fluid withdrawn from the rectification column, wherein the condensed overhead gas is used at least in part as a liquid reflux to the rectification column, wherein a first part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, compressed and reintroduced into the rectification column, and wherein a second part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, expanded and withdrawn from the air separation unit, wherein compressing the first part of the fluid which is used for cooling the overhead gas of the rectification column a compressor which is coupled to an electric motor via a first gearbox is used, in that for expanding the second part of the fluid which is used for cooling the overhead gas of the rectification column an expansion turbine which is coupled to an electric generator via a second gearbox is used, and in that the first gearbox and the second gearbox are provided with an identical design including identical reduction or multiplication ratios in the first gearbox and the second gearbox.
 2. The method according to claim 1, the identical reduction or multiplication ratios in the first gearbox and the second gearbox being provided by using at least one of identical sprocket diameters, identical numbers of teeth of sprockets, and identical shaft diameters in the first gearbox and the second gearbox.
 3. The method according to claim 1, wherein the first gearbox and the second gearbox are operated, or designed to be operated, in identical directions of rotation.
 4. The method according to claim 1, wherein the first part of the fluid which is used for cooling the overhead gas of the rectification column is a cryogenic liquid withdrawn from the rectification column at a first position, and wherein the second part of the fluid which is used for cooling the overhead gas of the rectification column is cryogenic liquid withdrawn from the rectification column at a second position.
 5. The method according to claim 4, wherein the first position is above the second position and/or wherein the second position corresponds to a position at the bottom of the rectification column.
 6. The method according to claim 1, wherein the first part of the fluid which is used for cooling the overhead gas of the rectification column has a higher nitrogen content than the second part.
 7. The method according to claim 1, wherein electric energy generated in the generator is at least in part used to operate the motor.
 8. The method according to claim 1, wherein the compressor and the expansion turbine are operated at identical speed settings.
 9. An air separation unit comprising a rectification column, the air separation unit being adapted to compress, cool and rectify feed air in the rectification column obtaining an overhead gas, wherein means are provided which are adapted to condense a part of the overhead gas of the rectification column using fluid withdrawn from the rectification column, wherein means are provided which are adapted to use the condensed overhead gas at least in part as a liquid reflux to the rectification column, wherein means are provided which are adapted to compress and reintroduce into the rectification column a first part of the fluid which is used for cooling the overhead gas of the rectification column after its use for cooling, and wherein means are provided which are adapted to expand and withdraw from the air separation unit a second part of the fluid which is used for cooling the overhead gas of the rectification column after its use for cooling, wherein compressing the first part of the fluid which is used for cooling the overhead gas of the rectification column a compressor which is coupled to an electric motor via a first gearbox is provided, in that for expanding the second part of the fluid which is used for cooling the overhead gas of the rectification column an expansion turbine which is coupled to an electric generator via a second gearbox is provided, and in that the first gearbox and the second gearbox are provided with an identical design including identical reduction or multiplication ratios in the first gearbox and the second gearbox.
 10. The method according to claim 9, the identical reduction or multiplication ratios in the first gearbox and the second gearbox being provided by using at least one of identical sprocket diameters, identical numbers of teeth of sprockets, and identical shaft diameters in the first gearbox and the second gearbox.
 11. The method according to claim 9, wherein the first gearbox and the second gearbox are operated, or designed to be operated, in identical directions of rotation.
 12. The air separation unit according to claim 9, adapted to perform a method for cryogenic separation of air, using an air separation unit comprising a rectification column, wherein feed air is compressed, cooled and rectified in the rectification column obtaining an overhead gas, wherein a part of the overhead gas of the rectification column is condensed using fluid withdrawn from the rectification column, wherein the condensed overhead gas is used at least in part as a liquid reflux to the rectification column, wherein a first part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, compressed and reintroduced into the rectification column, and wherein a second part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, expanded and withdrawn from the air separation unit, wherein compressing the first part of the fluid which is used for cooling the overhead gas of the rectification column a compressor which is coupled to an electric motor via a first gearbox is used, in that for expanding the second part of the fluid which is used for cooling the overhead gas of the rectification column an expansion turbine which is coupled to an electric generator via a second gearbox is used, and in that the first gearbox and the second gearbox are provided with an identical design including identical reduction or multiplication ratios in the first gearbox and the second gearbox. 